A silicon-on-insulator (SOI) substrate is a laminated substrate in which a silicon dioxide thin film, referred to as a buried oxide film, is formed, and furthermore, a silicon thin film, referred to as a silicon active layer, is formed thereon. The SOI can be used as a substrate of an optical integrated circuit; by etching the silicon active layer into a wire shape, it is possible to form an optical waveguide that has silicon as a core and has the buried oxide film and air as cladding. The silicon core may be embedded in silicon dioxide. (While light is guided along the waveguide, an electromagnetic field of the guided light is distributed so as to penetrate into the cladding from the core as a center. Therefore, the “waveguide” includes not only the core but also the cladding, and the “wire” refers to only the core of the waveguide. A similar situation applies in the following description. In addition “core width” refers to a distance between side faces of the core in a cross-section perpendicular to a direction of wave guiding, and “core height” refers to a distance between top and bottom faces of the core in a cross-section perpendicular to the direction of wave guiding.)
Combining micro elemental optical devices that have various basic functions and integrating them on a single SOI substrate can produce optical integrated circuits. Most elemental optical devices that form the optical integrated circuit are made of waveguides, which makes miniaturization of the devices relatively easy. Most basic elemental optical devices are optical waveguides themselves, and some of them are linear waveguides, bent waveguides, branched waveguides. By combining these waveguides as parts, it is possible to construct optical devices such as a directional coupler, an interferometer, and the like. Furthermore, by combining a wavelength filter with these optical devices, it is possible to construct an optical device such as wavelength multiplexers/demultiplexers, optical switches, or the like.
In the optical integrated circuit as mentioned above, the most basic elemental optical devices are optical waveguides. Usually, cross-sectional shape and size of a core of the optical waveguide are selected so that the optical waveguide has a single mode. However, cross-sectional shape and size of the core of a preferred waveguide in various devices are not necessarily the same (for example, refer to Patent Document 1 and Patent Document 2). If the optical waveguide is used for optical wiring, the most important characteristic of the optical waveguide is that waveguide loss is small. When it is etched into its wire shape, the silicon core suffers side wall roughness, which causes scattering loss. Therefore, it is desirable that the height of the core of a linear waveguide is small so that the area of side walls can be small. However, if the core is too thin (that is, the core height is too low), mode field can be to large, which will result in crease of propagation loss rather than decrease of a bent waveguides. Since an optical device such as a directional coupler includes many bent waveguides, in the case of optical circuits whose integration high, the high of a core should be large. Since thickness of a silicon active layer of an SOI substrate is uniform, in conventional technology, taking into account a trade-off between loss reduction and improvement of integration of the whole optical circuit on the substrate, the height of the core of the waveguide of the whole optical circuit has been selected.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2000-206352A
[Patent Document 2] JP Patent Kokai Publication No. JP-P2000-249856A